In recent years, toners have been required to have the following properties: i.e., a smaller particle diameter and hot offset resistance for giving higher quality to output images; low-temperature fixing ability for energy saving; and heat-resistant storage stability for resisting a high-temperature, high-humidity environment during storage or transportation after production. In particular, improvement in the low-temperature fixing ability is very important because power consumption for fixing occupies a large part of power consumption for the entire image forming process.
Hitherto, toners produced by a kneading and pulverizing method have been used. The toners produced by the kneading and pulverizing method have the following problems: their particle diameter is difficult to reduce; their amorphous shape and broad particle diameter distribution result in unsatisfactory quality of output images; and a large quantity of energy is required for fixing. When a wax (i.e., a release agent) is added to the toner in the kneading and pulverizing method for the purpose of improving a fixing ability, a large amount of the wax is present on toner surfaces because the kneaded product is cracked at an interface with the wax during pulverization. As a result, although a release effect is exhibited, the toner tends to deposit on a carrier, a photoconductor, and a blade (i.e., filming). Therefore, there is a problem that the toner is unsatisfactory from the viewpoint of entire performance.
In order to overcome the above-described problems associated with the kneading and pulverizing method, there has been proposed a method for producing a toner by a polymerization method. The toner produced by the polymerization method can be easily made to have a smaller particle diameter, can have a sharper particle size distribution than the toner produced by the pulverizing method, and can encapsulate a release agent. As the method for producing a toner by the polymerization method, there has been proposed a method for producing a toner using an elongation reaction product of urethane-modified polyester as a toner binder, for the purpose of improving the low-temperature fixing ability and the hot offset resistance (see, for example, Patent document 1).
Moreover, there has been proposed a method for producing a toner which is excellent in all of the heat-resistant storage stability, the low-temperature fixing ability, and the hot offset resistance, as well as excellent in powder flowability and transfer ability when the toner has a small particle diameter (see, for example, Patent documents 2 and 3).
Furthermore, there has been proposed a method for producing a toner, the method including an aging step for the purpose of producing a toner binder having a stable molecular weight distribution and achieving both of the low-temperature fixing ability and the hot offset resistance (see, for example, Patent documents 4 and 5).
However, these proposed techniques are unsatisfactory from the viewpoint of achieving high-level, low-temperature fixing ability which has been required in recent years.
For the purpose of achieving the low-temperature fixing ability at a high level, there has been proposed a toner which includes a wax and a resin including a crystalline polyester resin and has a sea-island, phase-separated structure due to incompatibility between the resin and the wax (see, for example, Patent document 6).
Moreover, there has been proposed a toner containing a crystalline polyester resin, a release agent, and a graft polymer (see, for example, Patent document 7).
These proposed techniques can achieve low temperature fixing because the crystalline polyester resin more rapidly melts than a non-crystalline polyester resin. Although the resultant toners can have both of the low-temperature fixing ability and the heat-resistant storage stability, there is a problem that, when the toner is used in high-speed apparatuses, stress applied to the toner in a developing device is increased to cause formation of toner aggregates and clogging of a cleaning doctor, resulting in white voids (white voids after transfer) of the toner on output toner images. Moreover, toners containing a crystalline polyester resin have a problem with formation of toner aggregates under a high-temperature, high-humidity environment.
Furthermore, in recent years, due to a demand for higher quality, there is a need for a toner being excellent in image gloss.
Accordingly, there is a need for provision of a polyester resin for a toner capable of forming a toner that achieves satisfactory low-temperature fixing ability and satisfactory high-temperature, high-humidity resistant storage stability at the same time, prevents white voids after transfer from occurring, and forms toner images excellent in blocking resistance and image gloss.